Viscous shear clutches are known in which a viscous fluid such as a silicone oil is used to transmit rotary motion from a drive shaft to a driven shaft via suitable coupling members. Such clutches rely upon mechanical adjustment of the coupling members to vary the torque transmitted by the fluid and suffer from the disadvantage that conditions of total disengagement and solid engagement are difficult to achieve. This latter disadvantage stems from the Newtonian flow properties of the viscous fluid, i.e. the fluid shears at a rate directly proportional to the applied shear stress, and consequently no matter how little stress is imposed by the coupling members, some shearing of the fluid will always occur.
Compositions are known in which the flow properties undergo variation when subjected to electric fields. These compositions, previously called `electroviscous fluids` but now more distinctively known as `electro-rheological fluids`, comprise slurries of finely divided hydrophilic solids in hydrophobic liquids. In the absence of an electric field these fluids behave in Newtonian fashion but, when an electric field is applied the fluid behaves approximately as a `Bingham Plastic` and no shearing whatsoever takes place for all shear stresses up to a maximum, known as the `yield-point`, the value of which depends upon the composition of the fluid and the electric field applied. For a given fluid, the yield-point is normally linearly dependent upon the voltage gradient applied above a threshold level, and may be defined as k(E--E.sub.o) where E and E.sub.o are the applied and the threshold voltage gradients respectively and k is the proportionality constant. When a shear stress greater than the yield-point is applied to the fluid, shearing occurs at a rate proportional to the difference between the yield-point and the stress.
The advantage of electrical control that can be achieved by using an electro-rheological fluid in a viscous shear clutch was first disclosed by W. M. Winslow in his U.S. Pat. No. 2,417,850, in which he described a simple disc type clutch in which the discs were separated by electro-rheological fluid. This type of clutch is capable of being locked into substantially solid engagement for stresses below the yield-point when an appropriate voltage is applied between the discs but, like all other viscous shear clutches, the natural viscosity of the fluid, even when all applied voltage is removed, still prevents total disengagement.